Hydrogen Isotope Separation Using a Metal–Organic Cage Built from Macrocycles

Abstract Porous materials that contain ultrafine pore apertures can separate hydrogen isotopes via kinetic quantum sieving (KQS). However, it is challenging to design materials with suitably narrow pores for KQS that also show good adsorption capacities and operate at practical temperatures. Here, we investigate a metal–organic cage (MOC) assembled from organic macrocycles and ZnII ions that exhibits narrow windows (<3.0 Å). Two polymorphs, referred to as 2α and 2β, were observed. Both polymorphs exhibit D2/H2 selectivity in the temperature range 30–100 K. At higher temperature (77 K), the D2 adsorption capacity of 2β increases to about 2.7 times that of 2α, along with a reasonable D2/H2 selectivity. Gas sorption analysis and thermal desorption spectroscopy suggest a gate‐opening effect of the MOCs pore aperture. This promotes KQS at temperatures above liquid nitrogen temperature, indicating that MOCs hold promise for hydrogen isotope separation in real industrial environments.


Gas sorption:
Nitrogen adsorption and desorption isotherms for all samples were collected at 77 K using an ASAP2020 volumetric adsorption analyzer (Micrometrics Instrument Corporation). Carbon dioxide and hydrogen isotherms were collected up to a pressure of 1200 mbar on a Micromeritics ASAP2020 at 77 K for hydrogen, or at 273 and 298 K for carbon dioxide. All samples were degassed at 80 °C for 15 hours under a dynamic vacuum (10−5 bar) before analysis.
A fully automated Sieverts apparatus (Autosorb-iQ2, Quantachrome Instruments) was used to perform the Ar isotherm at 87.3 K and the hydrogen cryogenic adsorption experiments. The calibration cell was an empty analysis carried out at the same temperature and pressure range of each experiment; corrections relating the sample volume and the nonlinearity of the adsorbate were made. Around 50 mg of each sample was activated at 80 °C for 2α and at 180 °C for 2β under vacuum for 12 h in order to remove any solvent molecules. A coupled cryocooler based on the Gifford−McMahon cycle was used to control the sample temperature. The cooling system permitted us to measure temperatures from 20 to 300 K with a temperature stability of <0.05 K.

Thermal desorption spectroscopy (TDS)
TDS experiments were carried out on an in-house designed device with about 2 mg of each sample. The sample holder is screwed tightly to a Cu block, which is surrounded by a heating spiral in the high vacuum chamber. The Cu block is connected to a flowing helium cryostat, allowing cooling below 20 K. All the samples were first loaded in the sample holder and activated under vacuum for 2 h at the temperature of 353 K for 2α and at 453 K for 2β. Then, the sample was exposed to a 10/200 mbar equimolar D2/H2 isotope mixture at different exposure temperatures (30, 50, 77, and 100 K) for 10 min. The remaining gas molecules were removed at the corresponding exposure temperatures until high vacuum was reached again. Afterwards, the sample was rapidly cooled down below 20 K. Then a linear heating ramp (0.1 K/s) was applied, the desorbing gas was continuously detected using a mass spectrometer (QMS), recognizing a pressure increase in the sample chamber when gas desorbs. The area under the desorption peak was proportional to the desorbing amount of gas, which can be quantified after careful calibration of the TDS apparatus.
A solid piece of a diluted Pd alloy Pd95Ce5 (~0.5 g) was used to calibrate the mass spectrometer signal. Before the calibration, the oxide layer of the alloy was removed by etching with aqua regia. Then the alloy was heated up to 600 K under a high vacuum to remove any hydrogen that might be absorbed during the etching procedure. Afterwards, it was exposed to 40 mbar pure H2 or pure D2 for 1.5-2.5 h at 350 K. As H and D were bound preferentially to the Cerium atoms at low exposure pressures, the alloy could be handled under ambient conditions for a short time. The alloy was weighed after being cooled down to room temperature. The mass difference between unloaded state and loaded state was equal to the mass uptake of hydrogen or deuterium, respectively. After weighing, the alloy was loaded in the chamber again, and then a 0.1 K/s heating ramp (room temperature (RT) to 600 K) was applied for a subsequent desorption spectrum. The obtained mass of gas is directly corresponded to the area under the desorption peak.

Computational details
An isolated molecule, extracted from experimental crystal structure of 2α, was optimized by GFN2-XTB method with D4 dispersion [2] model in gas phase with defaults for convergence. The optimized geometry was confirmed as a true minima by numerical harmonic frequency calculation without imaginary frequency. [3] Based on that, MD simulation was performed for 200 ps, in which 100 ps for equilibration and 100 ps for production with timestep of 2 fs, along with SHAKE restraints on all bonds, [4] in the NVT ensemble using the Berendsen thermostat [5] to maintain temperature of 298 K. Structures were dumped every 1 ps. The results will be very similar by using isolated molecule from 2β.
Pywindow [6] was used to calculate pore diameter and widow diameter of the molecular dynamics trajectories including 100 geometries obtained from xTB calculations.
Pore size distribution (PSD) histogram was calculated by Zeo++ [7] , it is advised to use probe radius similar to atomic radii for which one should expect the said 0.1 Å accuracy in peak positions. Therefore, we chose the probe radii below the half of the largest free sphere (Df) of the crystals' pore structure. As for the MeOH@2, 2α and 2β, the Df is 3.4, 2.0 and 2.6 Å, respectively. The probe radii chosen for MeOH@2 and 2β is 1.2 Å, and 0.97 Å is for 2α.

Water stability measurements
5 mg of 2α or 2β was added to a 5 mL vial containing 4 mL of deionized water to test hydrolytic stability of 2α and 2β,. The 2α and 2β samples were suspended in water without stirring at RT for 1, 2, and 5 days. Then, each 2α and 2β sample was removed by filtration and dried in air. PXRD patterns were recorded using the air-dried samples ( Figure S11), and NMR spectra were recorded after fully dissolving the air-dried samples in CDCl3 ( Figure S4 for 2α and Figure  S5 for 2β). As shown in Figures S4 and S5, the 1 H NMR spectra recorded after fully dissolving the air-dried 2α and 2β crystals in CDCl3 after being immersed in water for 1, 2, or 5 days are comparable, demonstrating that 2 is chemically stable in 2α and 2β in water at room temperature for at least 5 days The PXRD patterns that were recorded for 2α ( Figure S11a) and 2β (Figure S11b) after these samples were immersed in water for up to 5 days showed that some of the peaks shifted, and there were other differences in peak position. These differences indicate that 2α and 2β swell and change the structure slightly after being immersed in water. However, 2α and 2β remain crystalline, and their structures do not appear to collapse.

MeOH vapour sorption experiments
The large yellow block-shaped crystals of MeOH@2 formed during synthesis immediately lose solvent after being removed from MeOH and break up into smaller yellow crystalline powders in the air. The PXRD pattern of the air-dried MeOH@2 sample is not identical to the simulated PXRD pattern of MeOH@2; however, it closely matches the simulated PXRD pattern of the 2α structure ( Figure S12).
To further investigate the dynamic structural behaviour of 2α and 2β, we performed vapour sorption experiments using pure MeOH. For each MeOH vapour exposure test, an open 5 mL vial containing 15 mg of 2α or 2β was placed in a sealed 20 mL vial containing 2 mL of MeOH at RT. PXRD patterns of the MeOH-loaded samples were then collected for up to 5 days immediately after air-drying the samples on the PXRD plate. The PXRD pattern of 2α after being exposed to MeOH vapour for 2 days is similar to the PXRD of air-dried MeOH@2 ( Figure  S12a). By contrast, the PXRD patterns of 2β after being exposed to MeOH vapour appears to gradually transform into PXRD of air-dried MeOH@2 over the 5-days before the crystals begin to redissolve in MeOH in the vial (Figure 12b).     , there is little weight loss at 37.2 °C may be due to the unbound water, which is consistent to the NMR result that the methanol has been removed entirely in 2α.

Crystallography Report
( Figure S2) A temperature of ≥80 °C can remove all of the methanol in 2. Figure S7. DSC curves of 2α and 2β heated at a ramp rate of 10 °C/min under an N2 atmosphere.   Figure S10. PXRD patterns of 2 after and before testing gas isotherms: (a) PXRD patterns of 2α collected after and before testing H2 and D2 isotherms at 30 to 100 K. (b) PXRD patterns of 2β collected after and before testing H2 and D2 isotherms at 30 to 100 K. a b Figure S11. PXRD patterns that were collected after suspending (a) 2α in water and (b) 2β in water for 1, 2, and 5 days. The samples were collected by filtration and air-dried before the PXRD patterns were recorded. Figure S12. PXRD patterns of (a) 2α and (b) 2β after being exposed to MeOH vapour at RT for up to 5 d. The PXRDs of 2@MeOH after air-drying the sample at RT on the PXRD plate (red) and the simulated PXRD of MeOH@2 (black) are included in both plots.  Figure S15. N2 adsorption-desorption cycles of 2α recorded at 77 K. Solid symbols: adsorption; hollow symbols: desorption. The isotherms were cycled to investigate the reproducibility of the pressure-induced gating effect observed for 2α during N2 adsorption at 77 K. The N2 adsorption-desorption isotherm three times in a row using the same 2α sample. The three N2 adsorption isotherms all exhibited pressure-induced gating effects over the P/P0 range from 0.01 to 0.12, and have similar shapes and uptakes outside that range. In addition, the calculated BET surface areas for cycles 1, 2, and 3 were comparable (401, 418, and 398 m²/g, respectively) over the P/P0 range (0.06 to 0.32).  Figure S17. Isosteric heat of adsorption of H2 and D2 for 2α as a function of the adsorption amount. The isosteric heat of adsorption in 2α was calculated to be 3-7 kJ/mol for H2 and 4-8 kJ/mol for D2 using the Clausius-Clapeyron equation. A higher heat of adsorption is the reason for the higher D2 uptake. Interestingly, the enthalpy gradually rises with a higher adsorption amount, meaning that the host can adsorb gas molecules more easily at higher loadings. Meanwhile, the heat of adsorption in 2β was calculated to be 0.5-3.5 kJ/mol for both isotopes. The abnormal slope for 2α is attributed to the strong diffusion barrier due to the small pore aperture, which opens up at higher temperatures due to thermally-activated flexibility. Figure S18. Isosteric heat of adsorption of H2 and D2 for 2β as a function of the adsorption amount. Concerning the self-diffusion coefficient, for materials like MOCs, the diffusion limitation typically governs the adsorption/ desorption process. At low temperatures, gas molecules can only be adsorbed weakly on the outer surface; the gas can penetrate into the cavity at higher exposure temperatures. It is, therefore, difficult to calculate the self-diffusion coefficient for hydrogen isotopes. However, the maximum temperature of the TDS peaks correlates with the diffusion thru the apertures; that is, lower and higher maximum temperatures correspond to faster and slower diffusion, respectively. Thus, the higher maximum temperature of H2 (106 K 2α in and 92 K in 2β) compared to D2 (102 K 2α in and 86 K in 2β) indicates a slower H2 and faster D2 diffusion in MOC. Figure S19. H2 (Black) and D2 (Red) thermal desorption spectra of H2/D2 single gas at 10 mbar and 200 mbar for (a) 2α and (b) 2β. A laboratory-designed cryogenic thermal-desorption spectroscope (TDS) are utilized for determining the preferred H2 and D2 adsorption sites in 2α and 2β. TDS measurements were carried out by applying pure H2 and D2 atmospheres (10/200 mbar), respectively, under identical experimental conditions. The gas exposure was carried out at room temperature and cooling down to 20 K. The resulting TDS spectra obtained between 20 and 170 K. a b Figure S20. Molecules overlay for 2α (blue and cyan) and 2β (red), as generated using the Molecules Overlay tool in Mercury. H atoms are omitted for clarity.  [27] * D2/H2 mixture selectivity calculated by IAST